Baxi 105e Combi Boiler?

I have a Baxi Combi 105e which has been really great until now but has starting acting up and I think it needs something replacing in it? I really like this boiler and will try anything rather than replace it.

1) I noticed that when I ran the hot water tap when the heating was on that the boiler would cut out.

2) The heating would cut out (when it got up to as far as "50") and the temperature indicator lights would start flashing.

3) The water would also cut out if you attempted to run it after the boiler/heating cut out.

4) I have noticed that the lower the temperature outside, the better it definately seems to work?

5) I have also noticed that it also seems to be governed by how high the thermostat is set on the wall in my lounge e.g. the lower + the colder it is outside the better it works. The higher the thermostat is set and the higher the outside temperature is set the more it seems to play up.

I have recently had the diaphragm and nut replaced inside of it. It is working a little better, but nowhere near the level where it should be.

I have been putting up with this for months now and I am getting really fed up.

Any help or advice on this issue would be very gratefully received! ;-)